It is generally known that indigo is a vat dye which is insoluble in water. However, when the vat dye is reduced to the leuco form by addition of sodium hydrosulphide and sodium hydroxide, it is dissolved into a reduced form, but without any colour. Thus, to impart a colour, the leuco soluble form is subjected to the step of oxidation.
The process of dyeing with indigo is essentially known with respect to cotton yarn. Such a process of dyeing is carried out with the cotton yarn being in X open width or rope form. In the open width form total no. of ends required are arranged in a spread condition and are dyed and sized in one go to get a weaver's beam.
In the rope dyeing form, the yarns we formed into a rope. Generally, a single rope consists of approximately 350 to 400 yarns and passed through dyeing vats. After dyeing, the rope is opened to cause a separation of the yarns on reels and to provide the yarns in a side by side relationship to get a spread sheet.
U.S. Pat. No. 5,361,438 discloses a method of indigo dyeing so as to allow the yarns to be used in knitting machines. Specifically, U.S. Pat. No. 5,361,438 suggests a method to dye the yarn in skeins or hanks so that the dyed yarn can be wound on a spool rather than on a reel. In such a process, the yarn is in the skein form and placed on a feed and drawn roller and such that the skein is passed through a plurality of indigo dye baths followed by oxidation. The number of dye baths each followed by oxidation depends on the colour intensity.
U.S. Pat. No. T 100, 201 discloses a method for indigo dyeing of polyester fibers. In such a process the fibres are first coated with a polymeric salt so as to provide washdown and fading properties similar to indigo dyed cotton fibers.
Reverting to the continuous process of indigo dyeing of cotton yarn, the process consists in forming the yarns into ropes, each rope consisting of a bundle of yarns. Such ropes are introduced into a vat dye bath containing the indigo dye followed by oxidation. A plurality of vat dye baths may be provided and followed each time by the step of oxidation, the number of such repetitive steps of impregnation and oxidation being dependent on the intensity of colour to be dyed on the yarn.
The process of indigo dyeing on wool yarn by a batch process could perhaps be carried out by impregnating wool yarn in a spread form in a vat dye bath. The impregnated yarns are then hung in a loose form and subjected to the step of oxidation. Appropriate batch process for the indigo dyeing of woolen yarn has not been described in any literature, though a manner in which such a process could perhaps be carried out has been described hereinabove.